Studies on Photobacterium damselae subsp. damselae infecting marine finfish

Abstract

Marine fish farming is a very important activity of Indian aquaculture industry. The main marine fish species intensively cultured are cobia, asian seabass and pompano and several new marine fish species are being evaluated as potential candidates for aquaculture production. The intensive culture of these new fish species has favoured the appearance of several outbreaks with varied mortality rates. In recent years, Photobacterium damselae subsp. damselae has been repeatedly isolated from epizootic outbreaks affecting several cultured fish species in different geographical regions. In addition, this bacterial pathogen has been reported to cause diseases in humans, and for this reason, it may be considered as an agent of zoonoses. The present study was aimed to characterize and identify Photobacterium damselae subsp. damselae present in diseased cobia collected from Gulf of Mannar region of India. Photobacterium damselae subsp. damselae was isolated from gills, kidney, liver and spleen by using the thiosulfate citrate bile salts sucrose agar supplemented with 1.5 per cent NaCl medium. A total of 11 Photobacterium damselae subsp. damselae isolates were studied together with one reference strain. The biotyping and multiplex PCR analysis of ure C and 16S rRNA, 16S rDNA, Damselysin genes confirmed the phenotypic characterization of the isolates as Photobacterium damselae subsp. damselae. Experimental infection studies revealed Photobacterium damselae subsp. damselae was found in the internal organs of cobia and it showed pathogenicity to fish. The study reports the first time isolation of this bacterium from cultured cobia in Gulf of Mannar region, which warns us to pay more attention to fishery in this geographical area.